1. Field of the Invention
The present invention relates generally to optically addressed spatial light modulators and more particularly to perpendicular field optically addressed spatial light modulators.
2. Description of the Background Art
Recent demonstrations of perpendicular geometry GaAs/AlGaAs multiple quantum well (MQW) structures as optically addressed spatial light modulators (OASLM) have shown that these devices can offer high speed and high resolution.
These prior art devices use multiple quantum wells that have been made semi-insulating by ion implantation or chromium doping. An AC electric field is applied perpendicular to the plane of the wells at a rate faster than the dark screening time of the material. The surface of the quantum well is then exposed to a write beam that creates photocarriers which screen the applied field in the illuminated regions. The result is an internal electric field which is modulated in the same pattern as the write beam. The optical properties of the quantum well are then changed through the quantum confined Stark effect, and read out with a beam that is nearly resonant with the heavy hole exciton.
All MQW OASLMs to date have had deep traps distributed throughout the MQW region. These "bulk" traps have been thought necessary to decrease the dark conductivity of the MQW layers so that screening is largely due to photogenerated, and not background carriers. In addition, the bulk traps were included to restrict the lateral diffusion of photocarriers which degrades OASLM resolution. The use of these bulk traps, which were essential to the good operation of parallel field geometry MQW OASLMs, has been adopted by the prior art in the manufacture of perpendicular field MQW OASLMs.
Bowman et al., Appl Phys. Lett 65 (8), 22 Aug. 1994, pp 956-958 (the entirety of which is incorporated herein by reference, for all purposes), describe a perpendicular field MQW OASLM in which a proton-implanted semiconductor layer was sandwiched between two transparent electrodes. The bottom transparent electrode rested upon a glass substrate. The proton implanted semiconductor layer was photodiffracting and constructed from 75 periods of 10 nm GaAs quantum wells separated by 3.5 nm of Al.sub.x Ga.sub.1-x As barriers. Proton beam energy and fluence were controlled to generate a uniform defect density of 4.times.10.sup.16 cm.sup.-3 in the implanted semiconductor layer. After proton implantation, the dark resistivity of the semiconductor layer was approximately 2.times.10.sup.9 .OMEGA. cm. Samples with 10 mole percent aluminum in the barrier layers and samples with 30 mole percent aluminum in the barrier layer were made. The top electrode was insulated from the proton implanted semiconductor material by a dielectric layer. Additionally, the top and bottom surfaces of the semiconductor layer were clad with 30 nm of AlGaAs grown at a wafer temperature of 330.degree. C. and annealed at 580.degree. C. The material, described as "low temperature" AlGaAs, has been shown to have a dark resistivity of 10.sup.12 .OMEGA. cm for applied fields of less than 10.sup.5 V/cm. This perpendicular field MQW OASLM, while having good resolution, suffered from less than optimal speed and sensitivity. The less than desirable speed and sensitivity of previous perpendicular field MQW OASLMs reduces the efficiency of these devices and also limits their use. Additionally, the need for implantation increases the costs and complexity of manufacture.